(1) Field of the Invention
The present invention relates to a transmission output control apparatus for use in a mobile telephone and the like.
(2) Related Art
Conventionally for a mobile telephone, part of the transmission power to be supplied to an antenna is extracted from a directional coupler, so as to maintain the level of the transmission power from the power amplifying circuit to a certain standard power according to the power level of the extracted transmission power.
However, factors such as a metal plate being in the proximity of the antenna or the antenna being touched by a human hand cause the input impedance of the antenna to change. This leads to deterioration in the impedance matching between the antenna side and the transmission circuit side, causing a voltage standing wave to occur. As a result, the level of the transmission power extracted from the directional coupler will be inaccurate, preventing the output power level of the power amplifying circuit from being adjusted to the appropriate standard level.
In view of this, there has already been disclosed a technology of providing an isolator between an antenna and a directional coupler, thereby minimizing the effect of the voltage standing wave (see Japanese Laid-open patent application No. 2000-341143). According to the disclosed technology, when the matching between the transmission side and the antenna side is unfavorable, the antenna reflects back part of the transmission power from the power amplifying circuit, however the isolator obstructs the reflected power to suppress the voltage standing wave.
Generally speaking, however, such an isolator is expensive, and so is presenting one factor preventing the cost reduction of a mobile telephone.